The papermaking process includes providing pulp in a water solution along with adding various additives to form a continuous paper web. A series of operations in the papermaking process will transform the paper web into a finished product. The finished product may be used for paper, checks, notebooks, corrugated box, paper bag, packaging, paper board, cardboard, posterboard, or other applications.
During the papermaking process, on-line measurements of properties of paper material are taken to ensure the quality of the product. For instance, the on-line measurements may include basis weight, moisture content, caliper (i.e., thickness), and the like. The thickness of the paper material may be a predictor of strength, which influences stiffness in the finished product. Also, the thickness of the paper material may affect print quality on the finished product.
There are many methods currently used for measuring the caliper of the paper material. For instance, one method is to measure the caliper of the paper material by contacting devices. The contacting devices tend to place a contacting shoe along a top surface of the paper material. Unfortunately, there is a tendency for the contacting shoe to leave blemishes in the paper material or to cause flaws or tears in the paper material. Also, the contacting shoe may be affected by buildup of contaminants on contacting sensors, which requires constant maintenance of wiping and cleaning the contacting sensors of the contacting devices. Furthermore, the contacting sensors tend to wear out after time.
Another method is to measure the caliper of the paper material by non-contacting devices. The non-contacting devices may avoid the buildup of contaminants but face other problems. Some of these non-contacting devices use one laser source, which does not accurately measure the caliper due to the paper material moving towards the laser source. Other techniques may use two laser sources, one located on each side of the web. However, these techniques may also not be very accurate or have measurement errors if the web is not perpendicular to an incident laser light or the web experiences vibrations.